Transparent and/or translucent fluorescent color paper

ABSTRACT

The invention relates to a paper including at least a fluorescent pigment and which presents the following physical characteristics:  
     it is transparent and/or translucent,  
     it has an intensity of color (or saturation) c and a luminosity L, measured in CIELAB units in accordance with standard DIN 53237, such that the ratio of c to L is greater than 50%,  
     it has, for a given wavelength λmax, a coefficient of reflection ΔR greater than 100%, as measured on a paper placed on a white base,  
     as well as a preparation process for such a paper.

[0001] The present invention relates to a transparent and/or translucent paper having a fluorescent color, more particularly to a tracing paper having a fluorescent color, but also to other transparent or translucent papers such as greaseproof papers, papers made transparent by impregnation or so-called “crystal” papers, which have been given a fluorescent color by the means described below.

[0002] Since the terms used in the field of color, as in that of transparency, sometimes give rise to ambiguity, the inventors specify below the meaning which they allot to them:

FLUORESCENT COLORING MATERIALS

[0003] “Fluorescent coloring materials” are coloring materials, generally synthetic, which differ from conventional coloring materials in that they absorb certain frequencies of the luminous spectrum (in particular in the blue and the ultraviolet) and restore some of the luminous energy in narrow bands of the visible frequencies (above all the yellow, the orange and the red).

[0004] When they are lit by a polychromatic luminous source, these materials display in the spectrum of the visible frequencies a special reemission curve, characterized by the presence of an acute maximum the value of which can exceed 100% of the luminous intensity received for the considered frequency. At these frequencies, the material can thus emit more light than it receives. This is impossible for conventional coloring materials, which reflect or transmit only a part of the incident light. Only a perfect mirror achieves 100% reflection, a perfectly transparent medium 100% transmission.

[0005] The majority of the fluorescent coloring materials currently on the market greatly exceed 100% at maximum reemission. This property gives them extremely pure and luminous colors, and thus an extraordinary visual effect.

Dyes-Pigments

[0006] Regardless of their own chemical nature or their origin, coloring materials can be classified in two families depending on the form in which they are used:

[0007] when the coloring material is dissolved in the material to be colored, it is called a dye,

[0008] when the coloring material is used as a powder dispersed in the material to be colored, it is called a pigment.

[0009] Dyes may be soluble in water or in organic medium. The majority of pigments can be used equally well in an aqueous medium and in an organic medium.

[0010] There is currently no true fluorescent pigment on the market, but only fluorescent dyes, the most interesting of which are organosoluble. Producers of pigments circumvent the problem by introducing these dyes into resins which they polymerize before grinding them. The fine, brilliantly colored powder obtained is insoluble in water and in some solvents, and can thus be treated as a pigment. The fluorescent pigments involved in the invention are of this type.

[0011] The pigments industry, which is of interest to numerous industries but more particularly to the paints and inks industry, supplies colored products for which opacity is an essential quality. (When transparency is sought for color, the user turns to dyes which are naturally transparent). Any means that can be used to improve this opacity are employed by producers, in particular the increasing of the fineness of grinding. In all industries, pigments, and among them pigments having a fluorescent color, are thus regarded as opaque coloring materials.

Transparent-Translucent-Opaque Material

[0012] A transparent material is a material which light can cross without major loss. A colored transparent material is crossed without major loss by some frequencies of visible light, while other frequencies are strongly absorbed. The color transmitted by transparency is the resultant of the frequencies which cross the material.

[0013] When the material is not homogeneous and small foreign particles, opaque or transparent but having a different refractive index, are disseminated there, it is no longer perfectly transparent. From a certain thickness, it is no longer possible to distinguish the contours of an object situated on the other side because the light is interfered with while crossing it. The term translucent material is then used.

[0014] When the quantity of particles exceeds a certain threshold, the great majority of the light rays can no longer cross the material without being absorbed or reflected. The material is then opaque. This can happen when the number of foreign particles per unit of volume is high, or when the thickness of the translucent material to be crossed is fairly great.

Transparent and/or Translucent Paper

[0015] A paper will be regarded as transparent if, when a sheet of this paper is placed on a document comprising a text or drawing offering a minimum of contrast, the drawing can be distinguished or the text read through the paper.

[0016] Contrary to a widespread idea, a transparent paper can be intensely colored without losing its transparency as a black text remains perfectly visible when such a paper is placed over it.

[0017] A paper will be regarded as translucent if, under the same conditions, it does not allow the text to be read but the light zones and the dark zones of the drawing can be distinguished.

[0018] The industry of papers intended to be printed/written on has always sought to produce opaque papers, because the readability of the texts must not be hampered by the text of the following pages. Only some technical papers are transparent (crystal or greaseproof paper for packaging, tracing paper for the production of plans that can be superposed and reproduced by the diazo process). A person skilled in the art, in the field of the production of paper for printing/writing, thus did not seek, up until the present, to produce colored transparent papers. The invention of transparent and/or translucent papers having a vivid color and intended to be printed/written on was the subject of a patent filed by the applicant (PCT/FR/9800930). These tracing papers are characterized in that they have an intensity of color (or saturation) c and a luminosity L such that the ratio of c to L is more than 50%, or L is less than or equal to 60, or the ratio of c to L is greater than 50%, and L is less than or equal to 60. These papers are intended to be printed and/or written on. The paper described in this document also includes a fluorescent dye soluble in water, in solid form or in press size.

[0019] The present invention thus constitutes an improvement on this previous invention.

[0020] When a dye is used, the fluorescence obtained is actually not always satisfactory and the present invention seeks to improve the quality of the fluorescence.

[0021] Also known is an extra white transparent paper including a brightening agent in solid form in combination with an opacifying agent. Such a transparent paper is described in European patent application no. 0 919 664 filed by the applicant. This paper does not have an intense color. The optical brightening agent is a fluorescent dye which is soluble in water and which emits a bluish light when it is excited by daylight or under a UV lamp. It does not allow an intense color to be obtained. It does allow the luminance L of white papers to be increased and thus reduces the ratio c/L.

[0022] There is a description in document FR-A-2 348 317 (CIBA-GEIGY) of new optical brightening agents insoluble in aqueous medium which are used in suspension to effect a coating of an opaque paper. The aim of these brightening agents is to give the opaque paper an appearance of greater whiteness.

[0023] Document GB-A-1 369 202 (CIBA-GEIGY) describes a process for whitening an opaque paper using an acid solution of an optical brightening agent.

[0024] Document FR-A-2 660 944 describes a coating composition for opaque paper containing an optical brightening agent dissolved in organic polar solvents such as ethylene glycol, diethylene glycol, propylene glycol, dimethylformamide.

[0025] Document EP-A-0 712 960 describes a liquid composition containing an optical brightening agent dissolved in polyethylene glycol of low molecular weight in the range from 150 to 500. This composition is used to coat an opaque standard paper.

[0026] There currently exist in the trade opaque papers which have an intense fluorescent color. These papers are principally intended for the production of posters or labels for which transparency would be an awkward defect; producers thus endeavour to make them opaque. These papers are thus not transparent.

[0027] Thus there is currently no translucent and/or transparent paper which has a fluorescent color which is intense and which can be used in the field of printing and/or writing.

[0028] The aim of the present invention is thus to provide a translucent and/or transparent paper which has a fluorescent color which is intense and which can be used for printing and/or writing.

[0029] The present invention also aims to provide a production process for a translucent and/or transparent paper having an intense fluorescent color.

[0030] A person skilled in the art in the field of paper production knows that, to obtain a coloration of the latter, it is possible either to introduce dyes or pigments in solid form during the production of the paper, or to cover the paper with a composition containing dyes or pigments in suspension.

[0031] Since the paper is produced in suspension in water, it is convenient to use a hydrosoluble dye in solid form. Some of these dyes fix spontaneously on cellulose while others require the use of additives for their fixing. The pigments, insoluble in aqueous medium, can also be used so as to effect the coloration in solid form of the paper, but a fixing agent must be added which allows the pigments to be retained on the fibers. This does not prevent some loss of pigments in the waters discharged from the paper-making machine. Thus, to dye a paper, a person skilled in the art has on the one hand a range of transparent dyes soluble in water and on the other a range of opaque pigments insoluble in water.

[0032] A person skilled in the art who seeks to make paper having a fluorescent color can find in the trade fluorescent dyes which are soluble in water. These products are transparent but not very fluorescent; moreover, their range of colors is small, that is to say there are very few fluorescent dyes in the trade that are soluble in water. With these soluble fluorescent dyes, it is not possible to obtain a range of vivid and saturated, fluorescent colors having a c/L ratio greater than 50.

[0033] A person skilled in the art may also find in the trade organosoluble fluorescent dyes with vivid and varied colors. As these dyes cannot be used in water, they can be employed neither in solid form in the production of the paper nor on the surface in a hydrosoluble coating. They can be used solely in varnishes or inks in solvents that can be applied to paper, and because of their low reflectivity they do not give the paper a satisfactory color, in particular as regards fluorescence.

[0034] On the other hand, a person skilled in the art has at his disposal in the trade fluorescent pigments with very vivid and saturated colors in a varied range. As these are insoluble pigments and not soluble dyes, these products are regarded as opaque. To give the paper a fluorescent coloration, a person skilled in the art who wishes to realize a range of colors thus has at his disposal pigments which have varied fluorescent colors but are opaque.

[0035] Also known are paints based on fluorescent pigments which consist of transparent particles of resin. It is mentioned in technical sheet P.P.V. no. 166 of PEINTURES-Pigments-vernis that these pigments must be deposited on a white undercoat in order to obtain the maximum benefit from their luminosity and their purity and ensure that the light rays which have crossed the coat of fluorescent paint are not absorbed by the subjectile but reflected by it and re-cross the fluorescent coat in order to participate in the visual effect. According to the prior art, it is thus accepted that, to obtain a good fluorescence, it is necessary for the base to be as white and opaque as possible so as to reflect light to the maximum.

[0036] Consequently, a person skilled in the art believed, before the realization of the present invention, that it was not possible to obtain a transparent paper having a vivid fluorescent color and also in a range of varied colors. This prejudice is due to the fact that, if it is wished to obtain a range of varied fluorescent colors, a person skilled in the art is obliged to use fluorescent pigments which are not soluble in water, which can be deposited on the surface of the paper or incorporated without the body of the paper. A person skilled in the art believes that fluorescent pigments suppress the transparency of the paper. Another prejudice of a person skilled in the art is that, in order to obtain an effective fluorescence, it is essential for the base to be reflective and not transparent. Thus a person skilled in the art believes that it is not possible to obtain a transparent paper which has a certain fluorescence.

[0037] The present inventors surprisingly found that it was possible to give a transparent and/or translucent paper an intense fluorescent color by using fluorescent pigments, without these pigments significantly reducing the transparency of the paper finally obtained.

[0038] The present inventors also surprisingly found that by depositing an aqueous composition of fluorescent pigments by coating and/or printing on a transparent base, namely inter alia a standard transparent paper, an intense fluorescent color and a good transparency are obtained at one and the same time.

[0039] They also surprisingly found that that by introducing fluorescent pigments in solid form into the pulp of a transparent paper, the transparency of the paper obtained is not significantly reduced.

[0040] The present invention thus relates to a paper characterized in that it includes at least a fluorescent pigment and presents the following physical characteristics:

[0041] it is transparent and/or translucent,

[0042] it has an intensity of color (or saturation) c and a luminosity L, measured in CIELAB units in accordance with standard DIN 53237, such that the ratio of c to L is greater than 50%,

[0043] it has, for a given wavelength λ_(max), a coefficient of reflection ΔR greater than 100%, as measured on a paper placed on a white base.

[0044] The present invention also relates to a production process for a translucent and/or transparent paper having an intense fluorescent color, characterized in that:

[0045] a cellulose or a paper is chosen which is treated so as to obtain a transparent and/or translucent paper;

[0046] a pigment of the desired wavelength is chosen, which is mixed with the cellulose or which is deposited on the surface of the paper resulting from the treatment of the cellulose.

[0047] A paper according to the invention can be used for the writing on or printing on of, in particular, typographical characters.

[0048] According to one of the embodiments of the invention, the transparent or translucent paper receives on each side a surface coat including a fluorescent pigment insoluble in water.

[0049] The coat, at the moment of its application to the paper, can contain some 20 to 30% of fluorescent pigment by weight.

[0050] The dry coat resulting from this application can comprise from 1 to 10 g/m², preferably from 3 to 6 g/m² of fluorescent pigment on each side of the paper.

[0051] According to another embodiment of the present invention, a fluorescent pigment which is not soluble in water and opaque is introduced in solid form in a quantity of 5 to 20% expressed as dry weight of pigment relative to the dry weight of paper.

[0052] The paper according to the invention can be chosen from among the group formed by tracing paper, greaseproof paper, crystal (or “glassine”) paper, paper made transparent by impregnation.

[0053] The present invention, having regard to the figures and examples appended in a non-limitative manner, will allow a better understanding of how the invention can be put into practice.

[0054]FIG. 1 is a scale allowing convenient measurement of the transparency of a paper.

[0055]FIG. 2 is a schematic graph of the coefficient of transmission as a function of the wavelength of the reflected light.

[0056]FIG. 3 is a graph according to FIG. 2 obtained (curve A) by placing a paper according to Example 2 of the invention on a white base and obtained (curve B) by placing the paper according to Example 2 of the invention on a black base.

[0057] The paper according to the present invention is a paper which is transparent and/or translucent, which has an intensity of color, that is to say a saturation c, and a luminosity L measured in CIELAB units in accordance with standard DIN 53237, such that the ratio of c to L is greater than 50%, it has, for a given wavelength λ_(max), a coefficient of reflection ΔR greater than 100%, when the measurement is carried out on a paper resting on a white base (curve A).

[0058] The transparent and/or translucent paper can be a tracing paper obtained with cellulose fibers refined to a high Schöpper degree, at least greater than 80 or a paper made transparent by means of a chemical composition. The transparent and/or translucent paper can also be a paper known as “greaseproof paper” or “vegetable parchment” made transparent by immersing a paper having an opaque base in a bath of sulfuric acid which dissolves some of the cellulose of the fibers. This bath is followed by numerous rinsings with water which permits the elimination of the excess acid.

[0059] The transparent and/or translucent paper according to the present invention can also be a so-called “glassine” paper. Such a paper has a Bekk smoothness of more than 2000 s and is obtained by a high level of calendering of an opaque paper. A paper made transparent by any other chemical and/or mechanical means which is known or will be known in future can be provided.

[0060] The transparent and/or translucent paper according to the present invention has an intensity of color (or saturation c) and a luminosity L, measured in CIELAB units according to standard DIN 53237, such that the ratio c/L is greater than 50%. The paper according to the invention thus has an intense color.

[0061] The paper according to the present invention can consist of a transparent and/or translucent paper which comprises, on at least one of its sides, a surface coat of fluorescent pigment insoluble in water.

[0062] This coat of fluorescent pigment can be present on each of the sides of the paper.

[0063] The paper according to the invention has, for a given wavelength λ_(max), a coefficient of reflection ΔR greater than 100%.

[0064] The paper according to the invention is also characterized in that it can be obtained by deposition on a transparent paper of an aqueous composition including or not including a minute quantity of fillers.

[0065] The fluorescent pigments that can be used within the framework of the present invention can be chosen from among those marketed by the company CIBA under the MIGLO® mark, by the company RADIANT COLOR under the Jst® mark or the pigments of the SX range marketed by the company NOVAGLO.

[0066] The quantity of pigment deposited is regulated in order to preserve the transparency of the paper while still obtaining a desired vivid color which is sufficiently fluorescent. The quantity of pigment deposited on the paper per side is, according to the invention, between 3 and 6 g/m², expressed as dry weight. If the quantity is less than 3 g/m², the transparency of the paper is good, while the fluorescence and the intensity of the color are insufficient. If the quantity of pigment is greater than 6 g/m², the transparency of the paper is insufficient, but without any remarkable increase in the fluorescence and the intensity of color.

[0067] The transparency of the paper according to the invention can be conveniently measured with the help of a device designed by the applicant (represented in FIG. 1), in the following way:

[0068] Circles 2 with a diameter of 1 cm are traced on a transparent rigid strip 1 and filled with cross-ruled lines. A first circle 2 a is lightly cross-ruled so as to appear bright in color to the naked eye. The second circle 2 b, filled with closer cross-ruled lines, appears darker to the naked eye than does circle 2 a. The third circle 2 c, filled with closer cross-ruled lines, appears darker to the naked eye than does circle 2 b. The fourth circle 2 d, filled with closer cross-ruled lines, appears darker to the naked eye than does circle 2 c. The fourth circle 2 e, filled with closer cross-ruled lines, appears darker to the naked eye than does circle 2d, etc. The cross-ruling is regulated such that if the paper according to the invention 1 allows only circle 2 e to be seen, it will score 5, if it allows the clearer circle 2 d to be seen, it will score 4, etc., the paper which allows circle 2 a to appear being the best in terms of transparency and thus scoring 1.

[0069] The opacity of the paper can also be measured by the method according to a standard DIN 53147 or a standard such as JIS P 8138 or ISO 2469.

[0070] In FIG. 2, a schematic graph of the coefficient of reflection (or reflectance) has been represented on the ordinate and the wavelength of the reflected light (nm) on the abscissa. The reflectance or coefficient of reflection is measured by means of spectrocolorimetry using a DATACOLOR CS3 apparatus.

[0071] A curve containing a peak is obtained. The coefficient of reflection is greater in this zone and the curve forms a peak. The coefficient of reflection is greater than 100% in the case of a fluorescent pigment since there is emission of light by the fluorescent pigment.

[0072] In FIG. 3, a graph that was actually obtained has been represented, the curve A being obtained when the paper according to the invention is placed on a white-colored support and the curve B being obtained when the paper is placed on a black-colored support.

[0073] On curve A it is seen that the reflectance reaches almost 180%.

EXAMPLES Example 1

[0074] A 92 g/m² tracing paper is coated on each side with a Meyer bar. The formulation of the coat contains 22.5% expressed as dry weight of fluorescent pigment insoluble in water dispersed in an aqueous mixture containing a dispersing agent, a thickening agent and a wetting agent, and an acrylic binder. These agents and this binder are analogous to those used widely in the art for a paper coating. The pigment is a commercial-grade fluorescent yellow pigment, the average grain size of which is between 2 and 3 μm. 4.5 g/m² coat are deposited per side, expressed as dry weight, and the quantity of pigment contained in the coat expressed as dry weight is about 3 g/m² per side.

Examples 2 to 5

[0075] The same coats are deposited as in Example 1, but with a change of dye, as shown in the following Table 1. The measurements of the color of the papers obtained were carried out with a measurement apparatus marketed by the company HUNTERLAB.

[0076] Such an apparatus permits measurement of the Cartesian co-ordinates in three directions a, b, L of a point P of a color. Thus, point P is recorded as a function of the four colors blue, red, yellow and green. If a is positive, the color tends towards red, if a is negative, the color is greener. If b is positive, the color tends towards yellow, if b is negative, the color is bluer.

[0077] The HUNTERLAB apparatus also permits measurement of the polar coordinates of the point P. The distance between the center O and the point P is c and called chroma. It represents the intensity of the color. The angle H allows the point P to be situated relative to the four colors yellow, red, blue, green.

[0078] Transparency was measured by means of the device designed by the applicant, described above, and represented in FIG. 1. TABLE I Ex. no. Pigment L a b C H C/L % Transparency λ_(max) (nm) ΔR % 1 Yellow 96.02 −40.72 110.55 117.83 110.25 122.71 1 520 146 2 Orange-yellow 82.91 54.02 102.75 116.08 62.27 140.01 1 590 176 3 Orange-red 68.16 83.48 74.60 11.95 41.78 164.25 1 610 180 4 Pink 61.21 87.01 10.19 87.60 6.68 143.11 1 620 148 5 Green 82.93 −74.62 84.66 112.85 131.39 136.08 1 520 124

Example 6

[0079] Each of the two sides of a spool of 92 g/m² tracing paper is coated with a composition containing a small quantity of silica and having a fluorescent pigment content of 24% expressed as dry weight. The deposited coat expressed as dry weight is 5.5 g/m² per side.

Example 7

[0080] The same composition is applied as in Example 6, but to greaseproof paper. The coat expressed as dry weight as 7.4 g/m² per side, i.e. about 4 g/m² pigment per side.

Example 8

[0081] The same composition is applied as in Example 1, and under the same conditions, but to glassine paper. The coat expressed as dry weight is 3.5 g/m² pigment and binder per side, i.e. 2.45 g/m² pigment per side.

[0082] All the above examples allow a transparent paper having a vivid and fluorescent color to be obtained.

[0083] Although the coats of fluorescent pigment do not contain fillers, the paper according to the invention is easy to print on, which proves that the fillers are not necessary. This is an additional advantage of the invention, as in this way the formulation of the coats of fluorescent pigment are easier to realize. 

1. Paper characterized in that it that it includes at least a fluorescent pigment and presents the following physical characteristics: it is transparent and/or translucent, it has an intensity of color (or saturation) c and a luminosity L, measured in CIELAB units in accordance with standard DIN 53237, such that the ratio of c to L is greater than 50%, it has, for a given wavelength λmax, a coefficient of reflection ΔR greater than 100%, as measured on a paper placed on a white base.
 2. Paper according to claim 1, characterized in that consists of a transparent paper which includes, on at least one of its two sides, a surface coat of fluorescent pigment insoluble in water.
 3. Paper according to claim 2, characterized in that it includes a coat of fluorescent pigment insoluble in water on each of its sides.
 4. Paper according to one of claims 2 and 3, characterized in that the surface coat on each side comprises, expressed as dry weight, from 1 to 10 g/m², preferably from 3 to 6 g/m² of fluorescent pigment.
 5. Paper according to claim 1, characterized in that it includes, in solid form, a fluorescent pigment in a quantity of 5 to 20% expressed as dry weight of pigment relative to the dry weight of paper.
 6. Paper according to any one of claims 1 to 5, characterized in that the transparent paper is chosen from among the group formed by the tracing paper obtained with cellulose fibers having a degree of Schöpper refinement greater than 80, a greaseproof paper, a “crystal” or “glassine” paper, a paper made transparent by chemical means or other paper made transparent by chemical and/or mechanical means.
 7. Paper according to any one of claims 1 to 4, and 6, characterized in that it is obtained by deposition on a transparent paper of an aqueous composition containing a fluorescent pigment, in a quantity of about 20 to 30% expressed as wet weight.
 8. Paper according to claim 7, characterized in that it is obtained by deposition on a transparent paper of an aqueous composition including or not including a minute quantity of fillers.
 9. Paper according to any one of claims 1 to 4, 6 and 7, characterized in that it is obtained by deposition of an aqueous composition containing a fluorescent pigment by coating, heliographic, flexographic, offset or serigraphic printing.
 10. Application of a paper according to any one of claims 1 to 9 to printing and/or writing.
 11. Production process for a translucent and/or transparent paper having an intense fluorescent color, characterized in that: a cellulose or a paper is chosen which is treated so as to obtain a transparent and/or translucent paper; a pigment of the desired wavelength is chosen, which is mixed with the cellulose or which is deposited on the surface of the paper resulting from the treatment of the cellulose. 